Next-generation technical innovation in a semiconductor industry is progressing. When a semiconductor pattern is fabricated, a KrF excimer laser with a wavelength of 248 nanometers, furthermore, a liquid-immersion ArF excimer laser with a wavelength of 193 nanometers or the like is currently used. However, in aiming at achieving further finer lines, study has been made on processing with a target line width of 7 nanometers by utilizing extreme ultraviolet lithography for a wavelength of 13.5 nanometers.
When such a fine pattern is fabricated, mixing of only a limited amount of particles or metal ions in a working environment causes defects, and therefore a resist liquid to be used or the like is filtered by using a filter with a significantly small pore diameter such as several nanometers to several tens of nanometers in pore diameter in many cases. In such a filter, a filter made of polymer is used in order to prevent elution of metal ions in many cases.
As a raw material of such a filter, polypropylene has so far been used because of chemical stability and easiness of thermal processing. However, polypropylene has a disadvantage of being easily oxidatively decomposed, and therefore only a limited amount of fine particles produced by oxidative decomposition may be occasionally mixed into a filtrate, or an antioxidant added to polypropylene for preventing the oxidative decomposition may be occasionally mixed into the filtrate. In order to improve the above problem, polyethylene that is by far scarcely oxidizable than polypropylene has been frequently used.
Polyethylene is superior to polypropylene in view of difficulty in being oxidatively decomposed, but several hundred ppm of metal-containing substance is kneaded in the form of residues of a polymerization catalyst or a gel inhibitor in many cases. Such a metal-containing substance is ordinarily rarely eluted from polyethylene. However, if polyethylene is exposed into a liquid having high solubility, such as the resist liquid for a long period of time, but polyethylene is eluted little by little. Only a limited amount of the metal-containing substance has been known to cause the defect, particularly in a step in which processing is made at a short wavelength.
In JP-A-2008-179799, it is disclosed to use a polyethylene for high-purity chemical liquids produced using a Ziegler catalyst by a multistage polymerization method using specific components in order to reduce eluted components, the polyethylene having an effect to prevent the surface of a container from being roughened to increase in surface area. However, a filter cartridge made of a synthetic resin necessarily has an increased surface area due to use of a minute membrane or fibers as a raw material, and it was impossible to sufficiently suppress eluted components even if applying such measures.
JP-A-2000-317280 discloses an example of a pleated cartridge filter for the purpose of reducing eluted components. However, this document has no disclosure regarding a supporting material (spacer) that is essential for the preparation of a filter cartridge and has no investigation about a supporting material.